1. Field of the Invention
This invention relates to integrated circuit manufacture and more particularly to the fabrication of a metal silicide with enhanced silicidation properties using ion beam mixing.
2. Description of the Relevant Art
Fabrication of an integrated circuit involves numerous processing steps. After gate areas are defined upon a semiconductor substrate and impurity regions have been deposited within the substrate, interconnect routing is placed on the semiconductor topography and connected to contact areas to form an integrated circuit. The entire process of making an ohmic contact to the contact areas and routing interconnect material between ohmic contacts is described generally as "metallization". The term metallization is generic in its application and is derived from the origins of interconnect technology, where metals were the first conductors used. However, as the complexity of integrated circuits has increased, the complexity of the metallization composition has also increased.
In order to form highly conductive ohmic contacts in the connecting region or "window" between interconnect (generally aluminum) and underlying silicon (or polysilicon), it is oftentimes necessary to incorporate a layer of refractory metal at the juncture. The refractory metal, when subjected to high enough temperature, reacts in the contact window to form what is commonly called a refractory "silicide". Silicides are well known in the art and provide dependable silicon contact as well as low ohmic resistance.
Refractory metals generally require significant heating (600.degree. C. to 900.degree. C.) while in contact with the silicon in order to form a silicide region within the contact window. Often, such heating is required in order to break the residual (native) surface oxide barrier existing between the silicon and the refractory metal. Generally speaking, energy supplied by the heating step allows silicon to diffuse through the oxide and to the refractory metal thereby forming a silicide bond. The presence of oxide, however, does require significant heating in order to allow silicon diffusion and eventual oxide consumption. A trend in silicide formation is generally toward a two step heating process by which the contact window is heated (typically 600.degree.-650.degree. C.) to form a high resistivity silicide phase followed by heating (typically 800.degree.-900.degree. C.) to convert the silicide to a low resistivity phase. See, e.g., Miller, "Titanium Silicide Formation by RTA: Device Implications", 1st International RTP Conf., (RTP 1993), Sep. 8-10, 1993.
Subjecting the silicon substrate to high temperatures necessary for silicide formation offsets a major advantage of ion implantation as an alternative to diffusion predeposition. Ion implantation is usually more expensive than thermal deposition. However, an advantage of ion implantation is lower-temperature processing. By avoiding high thermal cycles upon the silicon, damage to the amorphous and crystalline portions within the silicon, as well as substrate topography, can be minimized. Unfortunately, high temperature, dual-step heating cycles necessary to form silicide, may cause processing damage and offset the advantages of ion implantation.
In an effort to reduce the temperatures necessary to form silicides, many researchers and manufacturers utilize a concept known as "ion beam mixing". Ion beam mixing (IBM) generally involves irradiating an interface between two materials with a beam of ions prior to an annealing step. Accordingly, the interface, and the demarcation between two materials at the interface juncture, can be "smeared" by the irradiating ions. Reaction between the two materials on either side of the smeared junction can thereby proceed at a lower temperature. IBM technology is well known and is generally described in Baumvol, "Aluminides and Silicides Formation by Ion Beam Mixing of Multilayers", Nuclear Instruments and Methods in Physics Research, (1993) pp. 98-104.
While IBM allows lower processing temperatures, silicon ions are generally used as the irradiating ion source. Use of silicon ions entails many disadvantages as a irradiating or "knocking" species. Most importantly, silicon ions of atomic weight 28 a.m.u., do not have sufficient mass to knock or dislodge a relatively heavy refractory metal atom, such as titanium from its bond position and through the interface region. The lightweight irradiating silicon ions generally bounce off the heavier titanium atoms. To compensate for their lighter weight, more silicon ions are needed during the irradiation process in order to move the titanium through the interface. Unfortunately, adding more silicon within the titanium interface region causes an abundance of silicon in the titanium and a deficiency of metal bonding sites, which leads to rejection of silicon moving through the interface from the underlying substrate. It is postulated that the ensuing effect is that of a silicide formation above the oxide leading to a lessening in the rate of reduction of residual (native) oxide. Any oxide which remains in the contact window resulting from such an effect will negatively impact the contact resistance between the interconnect and the underlying substrate. Further, it is postulated that the abundant silicon atoms are often present as interstitials which can enhance subsequent impurity (boron) diffusion from the source/drain regions. The added diffusion of boron from the source/drain resulting from silicon interstitial point defects therefore depletes concentration into the source/drain. Lessening of boron impurity results in an increase in P+ contact resistance at the impurity areas/junctions.
It is important to minimize the presence of any impurities such as an oxide in the contact window. It is also important to ensure quality ohmic contact exists regardless of the impurity deposited within the underlying silicon (or polysilicon). It is known that ohmic contact to arsenic-implanted silicon regions present significantly more problems than, for example, BF.sub.2 implanted contact regions. See, e.g., Liauh, et al., "Interfacial Reactions of Titanium Thin Films on Ion Implanted (001) Si", Nuclear Instruments and Methods in Physics Research", (1993) pp. 134-137. The presence of heavier atomic mass impurities, such as arsenic (having an atomic mass of 75), negatively impact silicide formation and effect the junction properties in the contact window. BF.sub.2 having an atomic mass of 49 appears not to cause the severe problems often associated with heavier arsenic. Thus, it is important to recognize this problem and, if possible, to enhance the silicide formation in contact windows having arsenic-implanted underlayers. It is postulated that arsenic creates significant lattice damage to the underlying silicon, and that the ensuing amorphous layer of silicon does not react well with the overlying refractory metal, especially due to the native oxide present at the interface. In order to offset this problem, it is important to present a silicide fabrication process which enhances the silicide formation in contact windows, and particularly in arsenic-implanted areas, while maintaining minimum silicon interstitials within the refractory metal and within the underlying silicon substrate. By compensating for the deleterious effects of arsenic implant and by avoiding silicon ion irradiation, and problems associated therewith, a significantly improved silicide process can be provided which overcomes the problems of conventional silicide methodologies.